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Currently, the CreateNativeWebModalManager() method declared inside the
web_modal component is defined outside of web_modal, which means linking it as a
shared component will fail. This is a layering violation.

This is now fixed for builds using Views, with constrained_window depending on
web_modal and web_modal providing definitions of all the symbols it declares.
Most unused implementations of CreateNativeWebModalManager() have been deleted,
with two remaining: create_native_web_modal_manager_cocoa.mm for Cocoa, and
test_create_native_web_modal_manager_cocoa.cc to prevent errors compiling
components_unittests, which isn't allowed to depend on the former file's
dependencies.

BUG=522654,492442

Review-Url: https://codereview.chromium.org/2087643003
Cr-Commit-Position: refs/heads/master@{#412730}